Ethylene manufacture entails the use of pyrolysis or "cracking" furnaces to manufacture ethylene from various gaseous and liquid petroleum feed stocks. Typical gaseous feed stocks include ethane, propane, butane and mixtures thereof. Typical liquid feed stocks include naphthas, kerosene, gas oil and crude oil.
Fouling of the cracking furnace coils and transfer line exchangers (TLEs) occurs due to coking and polymer deposition. The fouling problem probably is the major operational difficulty experienced in running an ethylene plant. Depending on deposition rate, ethylene furnaces must be periodically shut down for cleaning. In addition to periodic cleaning, "crash shut downs" are sometimes required due to dangerous increases in pressure or temperatures resulting from deposit build-up on furnace coils and TLEs. Cleaning operations are carried out either mechanically or by steam/air decoking.
Run lengths for ethylene furnaces average from one week to three months depending in part upon the rate of fouling of the furnace coils and TLEs. This fouling rate is in turn dependent upon the nature of the feed stock as well as upon furnace design and operating parameters. In general, however, heavier feed stocks and higher cracking severity result in an increased rate of furnace and TLE fouling.
In recent years, amine neutralized sulfonate treatments have been used in some ethylene plants to reduce furnace coil fouling. These compounds , however, have failed to prevent coking and fouling of TLEs immediately down stream of the furnace. The failure in respect of the TLEs may be due to premature degradation of the treatments in the ethylene furnace which sees temperatures in the range 1,000.degree. - 1,700.degree. F.
Applicants have now discovered that certain phosphate and phosphite mono and diesters will significantly reduce the fouling rate upon both the furnace coils and the transfer line exchangers. The use of these compounds as antifoulant additives in crude oil systems were previously described in U.S. Pat. Nos. 4,024,048, 4,024,049 and 4,024,050. The effectiveness of these compounds under the difficult conditions found in the ethylene cracking plant, however, has not been previously suggested.